High power electrical and electronic components continue to have an increasing demand for higher power dissipation within a relatively confined space. In order to provide for such higher power dissipation requirements while remaining suitably compact, several levels of thermal management are usually required at the device, sub-assembly and component level.
At the component level, various types of heat exchangers and heat sinks have been used that apply natural or forced convection or other cooling methods. A typical heat sink for electrical or electronic components is depicted in FIG. 1. As shown, the heat sink includes a heat spreader plate 10 to which metal fins 12 are attached. An electronic component is attached to face 14 of spreader plate 10 and a cooling fluid 16, such as air or water, is passed across fins 12 to dissipate the heat generated by the electronic component. For a given power level to be dissipated, the spreader plate size (i.e., area) and the fin length along the length of the cooling flow path can be calculated using known methods. Fin spacing and fin height are usually determined by known methods such as numerical modeling.
In demanding applications, the method of heat exchange is usually forced convection to the cooling fluid. In such systems, heat exchange can be improved by increasing the fin surface area exposed to the cooling fluid. This is accomplished by increasing the number of the fins per unit volume. However, there are limitations to achievable fin densities based upon manufacturing constraints and cooling fluid flow requirements.
Reticulated foams are also known in the art for their ability to conduct heat such as the metal foams disclosed in U.S. Pat. Nos. 3,616,841 and 3,946,039 to Walz, and the ceramic foams disclosed in U.S. Pat. No. 4,808,558 to Park et al. Metal foams have been sold under the trade name DUOCEL available from Energy Research and Generation, Inc., Oakland, Calif.
Until recently, metal and ceramic reticulated foams have not been adapted for use in heat sinks for dissipating waste heat from electronic components. However, these structures, especially when comprised of metal, make excellent heat exchangers because of their conductivity and their extremely high surface area to volume ratio. While earlier porous heat exchangers had up to 100 open cells per square inch, reticulated foam has up to 15,625 open cells per square inch. Reticulated foam is far more porous and has far more surface area per unit volume (1600 square feet/cubic foot) than heat exchangers having other structures. The pressure drop of fluids flowing through reticulated foam is also relatively low so that movement of a cooling fluid through the foam is practical.
Studies by Bastawros have now shown the efficacy of metallic foams in forced convection heat removal for cooling of electronics. See, Bastawros, A. -F., 1998, Effectiveness of Open-Cell Metallic Foams for High Power Electronic Cooling, ASME Conf. Proc. HTD-361-3/PID-3, 211-217, and Bastawros, A.-F., Evans, A. G. and Stone, H. A., 1998, Evaluation of Cellular Metal Heat Transfer Media, Harvard University report MECH 325, Cambridge, Mass. Bastawros demonstrated that the use of metallic foam improved heat removal rate with a moderate increase in the pressure drop. Bastawros' results were based on thermal and hydraulic measurements (on an open cell aluminum alloy foam having a pore size of 30 pores per inch) used in conjunction with a model based upon a bank of cylinders in cross-flow to understand the effect of various foam morphologies. The model prediction was extrapolated to examine the trade-off between heat removal and pressure drop. The measurements showed that a high performance cellular aluminum heat sink (i.e., aluminum foam) removed 2-3 times the usual heat flux removed by a pin-fin array with only a moderate increase in pressure drop.